The present invention, in some embodiments thereof, relates to a system for measuring reflectivity for multiple samples and angles of incidence, and more particularly, but not exclusively, to a surface plasmon resonance (SPR) system which allows simultaneous measurements of a two-dimensional array of samples.
In surface plasmon resonance, the intensity of light reflected from a thin metal surface (typically about 50 nm thick) decreases at a specific range of angles of the illumination relative to the metal surface. The illumination angle of greatest light absorption is sensitive to the refractive index of the material that is in direct contact with the other side of the metal layer. As shown in FIG. 1, the metal layer may be coated on the face of a prism, and the light passed through the prism to reflect from the back of the metal layer. FIG. 2 shows a typical reflectance as a function of angle of incidence, with a sharp dip at the SPR angle.
Typically, the front of the metal surface is exposed to a fluidic system that delivers the fluid containing samples or other materials automatically or manually. The fluidic system is usually made of one or more rubber or elastic polymer flow channels that are attached to the prism to generate a closed flow cell. This is shown in FIG. 3, where an elastic flow channel 300 is attached to a metal layer 106. The fluid flows from the left side and above the metal layer which can be coated with a ligand or capturing agents. The fluid itself can contain target molecules or analytes which are captured by the molecules or binding sites on the metal layer.
The addition of mass on the metal surface (as a consequence of binding for example) causes the SPR curve to shift on the angle axis, as shown in FIG. 4. This shift is measured as a function of time and displayed to the user as a measured signal. The shift can be interpreted as being proportional to the amount of molecules that are captured by the sensing surface.
By recording the shift of the SPR curve as a function of time, it is possible to measure the association and dissociation curves of molecules to other molecules. One possible method to extract reaction constants, rates and equilibrium is by fitting binding models to these curves. An example of this kind of curve is shown in FIG. 5.
U.S. Pat. No. 5,313,264, to Ivarsson, describes an optical biosensor system based on the evanescent wave phenomenon at total internal reflection, including SPR. The system reflects light over a range of angle of incidence from a set of sensing surfaces arranged in a row, horizontally. Using an anamorphic lens system, with different focal lengths in the horizontal and vertical planes, the reflected light is focused on a two-dimensional array of individual photodetectors, with different photodetectors in the same column representing different angles of incidence for the same sensing surface, and different photodetectors in the same row representing different sensing surfaces for the same angle of incidence.
Published European patent application EP0341928, to Finlan et al, describes an SPR system for detecting the distribution, in two dimensions, of DNA fragments undergoing electrophoresis on a sensing surface. A point or line of illuminating light is scanned over the surface. The illuminating light has a narrow range of angles of incidence that includes the SPR dip, and a photodetector, synchronized to the scan, measures the reflected light from each area on the surface, and detects a decrease in light reflected from those areas where a stronger SPR dip occurs.